The sort parameter expects an array:
julia> using VegaLite, DataFrames
julia> Summ_USCovid=DataFrame(:Month_Year => ["Apr 2020","May 2020","Apr 2021","May 2021"],:TotalCases => [100000.0,200000.0,400000,500000])
4×2 DataFrame
Row │ Month_Year TotalCases
│ String Float64
─────┼────────────────────────
1 │ Apr 2020 100000.0
2 │ May 2020 200000.0
3 │ Apr 2021 400000.0
4 │ May 2021 500000.0
julia> Summ_USCovid |> @vlplot(:line,x={:Month_Year,sort=["null"]},y=:TotalCases)
How did I found out?
The documentaion Sorting | Vega-Lite isn’t so helpfull in this case.
I don’t know if it is visible in a notebook (like in your screenshots) but when the VegaLite output is opened from a plain julia REPL just in a browser you can see “…” on top right:
There you can open the Vega Editor:
There the LOG is helping to find out what may be wrong.